POJ 3114 Countries in War

大意不再赘述。

思路：很简单、直接的一道题。首先通过强连通缩点，然后重建图，之后通过spfa找最小值。

重建图时：在一个国家之内的点，边权值为0，与其他的国家之间的边权值不变。

输出的时候有一个KD的BUG，害我找了N久。

#include <iostream>
#include <cstdlib>
#include <cstring>
#include <cstdio>
#include <queue>
using namespace std;

const int MAXN = 510;
const int MAXM = 510*510;
const int INF = 0x3f3f3f3f;

struct Edge
{
	int v, w;
	int next;
}edge[MAXM], edge2[MAXM];

int n, m;
int cnt, cnt2;
int scnt, top, tot;

int first[MAXN], low[MAXN], dfn[MAXN], stack[MAXN], ins[MAXN];
int d[MAXN];
int first2[MAXN];
int belong[MAXM];

char str[51] = "Nao e possivel entregar a carta";

void init()
{
	cnt = cnt2 = 0;
	scnt = top = tot = 0;
	memset(first, -1, sizeof(first));
	memset(first2, -1, sizeof(first2));
	memset(dfn, 0, sizeof(dfn));
	memset(ins, 0, sizeof(ins));
}

void read_graph(int u, int v, int w)
{
	edge[cnt].v = v, edge[cnt].w = w;
	edge[cnt].next = first[u], first[u] = cnt++;
}

void read_graph2(int u, int v, int w)
{
	edge2[cnt2].v = v, edge2[cnt2].w = w;
	edge2[cnt2].next = first2[u], first2[u] = cnt2++;
}

void dfs(int u)
{
    int v;
    dfn[u] = low[u] = ++tot;
    stack[top++] = u;
    ins[u] = 1;
    for(int e = first[u]; e != -1; e = edge[e].next)
    {
        v = edge[e].v;
        if(!dfn[v])
        {
            dfs(v);
            low[u] = min(low[u], low[v]);
        }
        else if(ins[v])
        {
            low[u] = min(low[u], dfn[v]);
        }
    }
    if(dfn[u] == low[u])
    {
        scnt++;
        do
        {
            v = stack[--top];
            belong[v] = scnt;
            ins[v] = 0;
        }while(u != v);
    }
}

void Tarjan()
{
    for(int v = 1; v <= n; v++) if(!dfn[v])
        dfs(v);
}

void spfa(int src)
{
	queue<int> q;
	bool inq[MAXN] = {0};
	for(int i = 1; i <= n; i++) d[i] = (i == src) ? 0:INF;
	q.push(src);
	while(!q.empty())
	{
		int x = q.front(); q.pop();
		inq[x] = 0;
		for(int e = first2[x]; e != -1; e = edge2[e].next)
		{
			int v = edge2[e].v , w = edge2[e].w;
			if(d[v] > d[x] + w)
			{
				d[v] = d[x] + w;
				if(!inq[v])
				{
					inq[v] = 1;
					q.push(v);
				}
			}
		}
	}
}

int read_case()
{
	init();
	scanf("%d", &n);
	if(!n) return 0;
	scanf("%d", &m);
	while(m--)
	{
		int u, v, w;
		scanf("%d%d%d", &u, &v, &w);
		read_graph(u, v, w);
	}
	return 1;
}

void build()
{
	Tarjan();
	for(int u = 1; u <= n; u++)
	{
		for(int e = first[u]; e != -1; e = edge[e].next)
		{
			int v = edge[e].v, w = edge[e].w;
			if(belong[u] != belong[v])
			{
				read_graph2(u, v, w);
			}
			else read_graph2(u, v, 0);
		}
	}
}

void solve()
{
	int q;
	build();
	scanf("%d", &q);
	while(q--)
	{
		int s, e;
		scanf("%d%d", &s, &e);
		spfa(s);
		//printf(d[e] != INF? "%d\n":"%s\n", d[e], str); KD的BUG，这样输出的永远是错误答案，害我找了N久。 
		if(d[e] != INF) printf("%d\n", d[e]);
		else printf("%s\n", str);
	}
	printf("\n");
}

int main()
{
	while(read_case())
	{
		solve();
	}
	return 0;
}